The figures, from the Office of National Statistics (ONS), follows a national trend of falling conception rates – with the first year of the coronavirus pandemic having a particular impact on teenage pregnancies.
The figures show there were 65 conceptions to girls aged under-18 in Northumberland in 2020 – down from 82 in 2019, and the lowest number since 1998, when records began.
They were among 12,576 under-18 pregnancies nationally in 2020.
The figures show there were 3,105 conceptions to women aged between 15 and 44 in the area during 2020 – up from 3,084 in 2019.
The British Pregnancy Advisory Service said the pandemic and measures introduced in response "have clearly had very different impacts" on the reproductive choices of women of different ages and backgrounds.
Spokeswoman Katherine O’Brien said: "Lockdown restrictions which prevented socialising with anyone other than members of your own household will have had a greater impact on women in younger age groups, who will be less likely to have been cohabiting with partners."
A fall in conceptions nationally has not led to a drop in births.
Dr James Tucker, head of health and life events analysis at the ONS, said: “Provisional births data shows the first half of 2021 saw a decrease in births, all of which would have been conceived in 2020 when conceptions decreased. The second half of 2021 saw an increase in births, relating to conceptions at the end of 2020 and beginning of 2021."